J BenComo

The management of imaging dose during image-guided radiotherapy: Report of the AAPM Task Group 75

Radiographic image guidance has emerged as the new paradigm for patient positioning, target localization, and external beam alignment in radiotherapy. Although widely varied in modality and method, all radiographic guidance techniques have one thing in common--they can give a significant radiation dose to the patient. As with all medical uses of ionizing radiation, the general view is that this exposure should be carefully managed. The philosophy for dose management adopted by the diagnostic imaging community is summarized by the acronym ALARA, i.e., as low as reasonably achievable. But unlike the general situation with diagnostic imaging and image-guided surgery, image-guided radiotherapy (IGRT) adds the imaging dose to an already high level of therapeutic radiation. There is furthermore an interplay between increased imaging and improved therapeutic dose conformity that suggests the possibility of optimizing rather than simply minimizing the imaging dose. For this reason, the management of imaging dose during radiotherapy is a different problem than its management during routine diagnostic or image-guided surgical procedures. The imaging dose received as part of a radiotherapy treatment has long been regarded as negligible and thus has been quantified in a fairly loose manner. On the other hand, radiation oncologists examine the therapy dose distribution in minute detail. The introduction of more intensive imaging procedures for IGRT now obligates the clinician to evaluate therapeutic and imaging doses in a more balanced manner. This task group is charged with addressing the issue of radiation dose delivered via image guidance techniques during radiotherapy. The group has developed this charge into three objectives: (1) Compile an overview of image-guidance techniques and their associated radiation dose levels, to provide the clinician using a particular set of image guidance techniques with enough data to estimate the total diagnostic dose for a specific treatment scenario, (2) identify ways to reduce the total imaging dose without sacrificing essential imaging information, and (3) recommend optimization strategies to trade off imaging dose with improvements in therapeutic dose delivery. The end goal is to enable the design of image guidance regimens that are as effective and efficient as possible.

Characterization and clinical evaluation of a novel IMRT quality assurance system

Intensity‐modulated radiation therapy (IMRT) is a complex procedure that involves the delivery of complex intensity patterns from various gantry angles. Due to the complexity of the treatment plans, the standard care is to perform measurement‐based, patient‐specific quality assurance (QA). IMRT QA is traditionally done with film for relative dose in a plane and with an ion chamber for absolute dose. This is a laborious and time‐consuming process. In this work, we characterized, commissioned, and evaluated the QA capabilities of a novel commercial IMRT device, Delta4, (ScandiDos, Uppsala, Sweden). This device consists of diode matrices in two orthogonal planes inserted in a cylindrical acrylic phantom that is 22 cm in diameter. Although the system has detectors in only two planes, it provides a novel interpolation algorithm that is capable of estimating doses at points where no detectors are present. Each diode is sampled per beam pulse so that the dose distribution can be evaluated on segment‐by‐segment, beam‐by‐beam, or as a composite plan from a single set of measurements. The end user can calibrate the system to perform absolute dosimetry, eliminating the need for additional ion chamber measurements. The patients IMRT plan is imported into the device over the hospital LAN and the results of the measurements can be displayed as gamma profiles, distance‐to‐agreement maps, dose difference maps, or the measured dose distribution can be superimposed on the patients anatomy to display an as‐delivered plan. We evaluated the systems reproducibility, stability, pulse‐rate dependence, dose‐rate dependence, angular dependence, linearity of dose response, and energy response using carefully planned measurements. We also validated the systems interpolation algorithm by measuring a complex dose distribution from an IMRT treatment. Several simple and complex isodose distributions planned using a treatment planning system were delivered to the QA device; the planned and measured dose distributions were then compared and analyzed. In addition, the dose distributions measured by conventional IMRT QA, which uses an ion chamber and film, were compared. We found that the Delta4 device is accurate and reproducible and that its interpolation algorithm is valid. In addition, the supplied software and network interface allow a streamlined IMRT QA process. PACS number: 87.56Fc

Quality assurance for nonradiographic radiotherapy localization and positioning systems: Report of Task Group 147

New technologies continue to be developed to improve the practice of radiation therapy. As several of these technologies have been implemented clinically, the Therapy Committee and the Quality Assurance and Outcomes Improvement Subcommittee of the American Association of Physicists in Medicine commissioned Task Group 147 to review the current nonradiographic technologies used for localization and tracking in radiotherapy. The specific charge of this task group was to make recommendations about the use of nonradiographic methods of localization, specifically; radiofrequency, infrared, laser, and video based patient localization and monitoring systems. The charge of this task group was to review the current use of these technologies and to write quality assurance guidelines for the use of these technologies in the clinical setting. Recommendations include testing of equipment for initial installation as well as ongoing quality assurance. As the equipment included in this task group continues to evolve, both in the type and sophistication of technology and in level of integration with treatment devices, some of the details of how one would conduct such testing will also continue to evolve. This task group, therefore, is focused on providing recommendations on the use of this equipment rather than on the equipment itself, and should be adaptable to each users situation in helping develop a comprehensive quality assurance program.

Magnification mammography: evaluation of screen-film and xeroradiographic techniques.

An x-ray unit designed for conventional nonmagnification and magnification mammography has been evaluated in terms of image quality and corresponding radiation exposure levels. The technical advantages of the radiographic magnification technique can result in improved image quality and reduction of the recording-system noise. The microfocal spot allows 1.5 x magnification mammograms with minimal geometric unsharpness. However, the magnification technique requires an increased radiation dose to the breast, compared to conventional nonmagnification techniques. An additional radiation dose may be required for screen-film magnification views because of reciprocity law failure due to long exposure times. The increased-dose limitation and the small dimensions of the recording-system cassettes have precluded the use of magnification in place of nonmagnified images for routine mammographic examination. The magnification technique has proved to be beneficial in selected cases.

Verification of the accuracy of 3D calculations of breast dose during tangential irradiation: measurements in a breast phantom

This report specifically describes the use of a unique anthropomorphic breast phantom to validate the accuracy of three‐dimensional dose calculations performed by a commercial treatment‐planning system for intact‐breast tangential irradiation. The accuracy of monitor‐unit calculations has been corroborated using ionization chamber measurements made in this phantom. Measured doses have been compared to those calculated from a variety of treatment plans. The treatment plans utilized a 6‐MV x‐ray beam and incorporated a variety of field configurations and wedge combinations. Dose measurements at several clinically relevant points within the breast phantom have confirmed the accuracy of calculated doses generated from the variety of treatment plans. Overall agreement between measurements and calculations averaged 0.998±0.009. These results indicate that the dose per monitor‐unit calculations performed by the treatment‐planning system can be confidently utilized in the fulfillment of clinical dose prescriptions. PACS number(s): 87.53.–j, 87.66.–a

Code of Ethics for the American Association of Physicists in Medicine: report of Task Group 109.

A comprehensive Code of Ethics for the members of the American Association of Physicists in Medicine (AAPM) is presented as the report of Task Group 109 which consolidates previous AAPM ethics policies into a unified document. The membership of the AAPM is increasingly diverse. Prior existing AAPM ethics polices were applicable specifically to medical physicists, and did not encompass other types of members such as health physicists, regulators, corporate affiliates, physicians, scientists, engineers, those in training, or other health care professionals. Prior AAPM ethics policies did not specifically address research, education, or business ethics. The Ethics Guidelines of this new Code of Ethics have four major sections: professional conduct, research ethics, education ethics, and business ethics. Some elements of each major section may be duplicated in other sections, so that readers interested in a particular aspect of the code do not need to read the entire document for all relevant information. The prior Complaint Procedure has also been incorporated into this Code of Ethics. This Code of Ethics (PP 24-A) replaces the following AAPM policies: Ethical Guidelines for Vacating a Position (PP 4-B); Ethical Guidelines for Reviewing the Work of Another Physicist (PP 5-C); Guidelines for Ethical Practice for Medical Physicists (PP 8-D); and Ethics Complaint Procedure (PP 21-A). The AAPM Board of Directors approved this Code or Ethics on July 31, 2008.

Logistic representation of the sensitometric response of screen–film systems: Empirical validation

An empirical logistic model that linearizes the sensitometric response data of screen-film systems over the entire dynamic range of exposures is presented. This linearization is evident when the net optical densities are scaled as fractions of the net saturation density and plotted on commercial logit, probit, or double-log paper, except under conditions of reciprocity law failure. Weighted linear regression analysis shows that the intercept, but not the slope, depends on the screen-film system used. Previous work indicates that the slope is also independent of development time and photon energy. The model is verified through an analysis of tabulated sensitometric data published by the Center for Devices and Radiological Health.

An examination of errors in characteristic curve measurements of radiographic screen/film systems.

The precision and accuracy achieved in the measurement of characteristic curves for radiographic screen/film systems is quantitatively investigated for three techniques: inverse square, kVp bootstrap, and step-wedge bootstrap. Precision of all techniques is generally better than +/- 1.5% while the agreement among all intensity-scale techniques is better than 2% over the useful exposure latitude. However, the accuracy of the sensitometry will depend on several factors, including linearity and energy dependence of the calibration instrument, that may introduce larger errors. Comparisons of time-scale and intensity-scale methods are made and a means of measuring reciprocity law failure is demonstrated.

TU‐D‐M100F‐04: Characterisation, Commissioning and Evaluation of DELTA4 IMRT QA System

Purpose: Characterize, commission and evaluate a dual plane diode matrix IMRT QA device. Method and Materials: A novel device consisting of diode matrices in two orthogonal planes inserted in a cylindrical acrylic phantom of 22cm diameter is characterized, commissioned and evaluated for radiotherapyquality assurance. The system interfaces readily with a networked computer making the whole IMRT QA process very efficient in multi accelerator and multi physicist department. . It detects charge per accelerator pulse, computes and displays measured dose distribution in 3D space. The temperature dependence of the diode is corrected. The precision, stability, pulse rate dependence, dose rate dependence, angular dependence, linear response, energy response of the system and the calculation accuracy at non detector locations are evaluated in addition to comparing multiple simple and complex iso‐dose distributions from TPS to measured distributions. The software readily analyses dose profiles in any orientations, %dose, DTA and gamma index of the entire 3D distributions. Results: The precision and the day‐to‐day reproducibility of measured data of a single field are excellent, making additional ion chamber measurement unnecessary. The measured data indicated excellent dose linearity and pulse rate independence. Comparison of simple and complex treatment plans with delivered treatment showed good agreement considering the error bars. Conclusions: The Delta 4 system is highly efficient, accurate and reproducible. The instantaneous and automatic data acquisition combined with the error analysis, report and database capability built into the system make it easy, convenient and efficient to use in a busy clinic. Conflict of Interest Statement: One of the co‐author is President and CEO of ScandiDos AB Company, which supplied Delta4 at no cost for evaluation.

A method of evaluating and minimizing geometric unsharpness for mammographic X-ray units.

A method for measuring and minimizing the effects of geometric unsharpness in mammography involves using a star resolution pattern to determine the equivalent focal spot size of mammographic x-ray units. With this measurement, the limit of resolution at any plane within the breast and the focal spot-to-object distance necessary to obtain the desired limit of resolution are determined. Six mammographic x-ray units were evaluated with this technique. Results show that the resolution capability of four of these units is limited by geometric unsharpness, such that the resolution capability of present mammographic recording systems is not fully utilized.